There has been a great increase in recent years in the amount of research and development carried out in optical devices mostly because of the development of optical communication systems and various laser-related systems such as cutting and trimming systems and laser recording systems. The advantages of optical communication systems are well known and include high bandwidth, high rate of information transmission, relative ease of optical multiplexing (transmitting several optical wavelengths on the same optical fiber), and use of glass fibers instead of copper transmission cables. Optical communication systems promise to provide high capacity communication at a much lower cost than traditional copper-based systems.
An optical transmission system is generally made up of an optical source (a light-emitting diode or laser), optical fibers, and a detector. Optical fibers are generally made out of silica glass with various kinds of index of refraction profiles used to insure proper propagation of light down the fiber. Generally, information is transmitted in optical fibers in the form of pulses (so-called pulse code modulation). The bandwidth and repeater spacings of an optical communication system depend not only on losses in the optical fiber but also on the material dispersion of the fiber.
Optical losses in the transmission system may be minimized by judicial use of anti-reflection coatings on surfaces that interface device material and air. In particular, anti-reflection coatings on light-emitting diodes (LED) used in optical communication systems insures greater output of radiation from the LED and into the optical fiber used in the transmission system. Also, the effectiveness of detectors are greatly increased by judicial use of anti-reflection coatings to minimize losses.
Although anti-reflection coatings can be used on a variety of optical devices used in various spectral regions, they are particularly useful for devices used in optical communications in the wavelength region from about 1.2 to 1.6 .mu.m. There are a number of advantages to the use of light wavelengths in this region. First of all, with many types of fibers, particularly fibers containing silica glass or phosphosilicate glass, losses are minimal in this wavelength region and material dispersion is very low in this region (see, for example, "Zero Material Dispersion in Optical Fibers", by D. N. Payne and W. A. Gambling, Electronic Letters 11 No. 8, page 176-178 (April, 1975) and "Photodiodes for Use in Long-Wavelength Fiber-Optic Communication Systems", by S. R. Forrest to appear in the November 1982 issue of Laser Focus).
Because of the low loss and low material dispersion, optical communication systems operating in the 1.2 to 1.6 .mu.m wavelength region (henceforth referred to as the 1.3 .mu.m region) have the advantage of greater bandwidth and greater repeater spacings. For these reasons, device development (particularly LED and detector development) has been concentrated on this wavelength region. Most such devices use indium phoshide as a substrate material because it is transparent in this wavelength region and the crystal structure provides an excellent lattice match to various compound semiconductors such as indium gallium arsenide and indium gallium arsenide phosphide. For front-emitting diodes and photodiodes, light usually enters or leaves the device through the indium phosphide substrate. For lasers or edge-emitting diodes, light leaves the device through the indium gallium arsenide phosphide active layer. Particularly useful is an anti-reflection coating which would minimize reflection, have extremely low losses, have a thermal expansion coefficient close to that of indium phosphide or indium gallium arsenide phosphide and be applied at reasonable temperatures so as to avoid excessive thermal expansion mismatch. The commercially important indium gallium arsenide phosphide compounds are those that are lattice matched to indium phosphide. Such compounds do not necessarily include all four elements (e.g., indium gallium arsenide).